1. Field of the Invention
The present invention relates generally to lithographic projection apparatus and particularly to lithographic projection apparatus including an interferometer.
2. Description of the Related Art
The term xe2x80x9cpatterning structurexe2x80x9d should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d has also been used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask table for holding a mask. The concept of a mask is well known in lithography, and its includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The mask table ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask table and mask; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
To reduce the size of features that can be imaged, it is desirable to reduce the wavelength of the illumination radiation. Wavelengths of less than 180 nm are therefore employed, for example 157 nm or 126 nm. However, such wavelengths are strongly absorbed by normal atmospheric air leading to unacceptable loss of intensity as the beam traverses the apparatus. In order to solve this problem, it has previously been proposed to flush the apparatus with a flow of gas, the gas being substantially transparent to the illumination wavelength, e.g. nitrogen (N2).
Lithographic projection apparatus may comprise interferometric displacement measuring means, which are used to accurately determine the position of movable tables, such as mask or substrate tables. These means measure the optical path length (geometrical distancexc3x97refractive index) to the movable tables using measurement beams of coherent monochromatic radiation. The measuring means are very sensitive to variations in pressure and temperature and in the composition of the medium that the measurement beams traverse through. All three of these variables affect the refractive index of the medium. Typically, in order to account for the variations in refractive index caused by temperature and pressure fluctuations, a second harmonic interferometric device is used. More information with regard to such second harmonic interferometric devices which are capable of compensating for temperature and pressure fluctuations, can be found, for example, in U.S. Pat. No. 5,404,222, which is incorporated herein by reference.
The second harmonic interferometer can, alternatively, compensate for variations in the composition of the medium. However, this second interferometric device cannot simultaneously account for the variations in pressure and temperature and in the composition of the medium.
Some spaces of the projection apparatus may be flushed with a purge gas in order to remove any gas, such as oxygen or water, which absorbs radiation at the wavelength of the projection beam of radiation. The inventors have found that if the gas used to purge the system enters the area where the interferometric displacement measuring means operate, the refractive index in these areas changes and the position measurements are affected.
In order to keep the measuring means operating to the high degree of accuracy that is required, any variation from the refractive index of said medium must be avoided.
One aspect of the present invention provides a lithographic projection apparatus in which escapage of the purge gas does not influence the interferometric displacement measuring means in part by use of an interferometer operating at a wavelength xcex1 to measure the position of said substrate table or the position of a table which is a part of said patterning structure, a purge gas source to supply purge gas to a space, to displace therefrom ambient air, said space accommodating at least one of at least a part of said substrate table and at least a part of said table which is a part of said patterning structure, wherein said purge gas is substantially non-absorbent of said projection beam of radiation and has a refractive index at a wavelength xcex1 which is substantially the same as that of air when measured at the same wavelength, temperature and pressure.
The inventors have found that by flushing, for example, the mask and substrate stages, which typically comprise movable mask and substrate tables respectively, with a specific gaseous composition having a refractive index identical to that of air under the same measuring conditions, the interferometric displacement measuring means are able to operate to the required degree of accuracy, while permitting the use of radiation having a wavelength of 180 nm or less.
According to a further embodiment of the invention, there is provided an apparatus as specified above, which further comprises a second harmonic interferometric measuring means operating at wavelengths xcex2 and xcex3 for adjusting the measurements of said interferometric displacement measuring means to substantially eliminate the effects of variation in pressure and temperature, and
wherein said purge gas comprises three or more different components, each component having refractivities at the wavelengths xcex2 and xcex3 such that the following equations are substantially fulfilled:                                                         ∑              j                        k                    ⁢                                    F              j                        ⁢                          α              j1                                      =                  α          a1                                    (        1        )                                                                    ∑              j                        k                    ⁢                                    F              j                        ⁡                          (                                                α                  j3                                -                                  α                  j2                                            )                                      =                              α            a3                    -                      α            a2                                              (        2        )            
wherein Fj is the fraction by volume of component j in the purge gas, which purge gas contains a total of k components, xcex1j1 is the refractivity of component j at a wavelength xcex1, xcex1j2 is the refractivity of component j at a wavelength xcex2, xcex1j3 is the refractivity of component j at a wavelength xcex3, xcex1a1 is the refractivity of air at a wavelength xcex1, xcex1a2 is the refractivity of air at a wavelength xcex2 and xcex1a3 is the refractivity of air at a wavelength xcex3; and wherein:                                                         ∑              j                        k                    ⁢                      F            j                          =        1.                            (        3        )            
Where a second harmonic interferometer is present in the apparatus, the simple adjustment of the refractive index of the purge gas to that of air at the wavelength of operation of the displacement measuring interferometer, is not sufficient to overcome the errors in the displacement measurement caused by purge gas leakage. The present inventors have therefore devised a purge gas composition which comprises at least three different components, each component typically having significantly differing refractivities (wherein the refractivity is defined as the refractive index-1). With proper choice of gases, such that the gases fulfil, or substantially fulfil the equations given above, the compositional variation of the purge gas will have no, or substantially no effect on the measurements of either interferometer. Thus accurate positional measurements, which take into account variations in temperature, pressure and leakage of purge gas, may be obtained.
According to a further aspect of the invention there is provided a lithographic apparatus as specified in the opening paragraph which is characterized by:
an interferometric displacement measuring means operating at a wavelength xcex1 for measuring the position of said substrate table or the position of a table which is a part of said patterning structure;
a second harmonic interferometric measuring means operating at wavelengths xcex2 and xcex3 for adjusting the measurements of the said interferometric displacement measuring means to substantially eliminate the effects of variation in pressure and temperature;
flushing gas means for supplying purge gas to a space, to displace therefrom ambient air, said space accommodating at least a part of said substrate table and/or at least a part of said table which is a part of said patterning structure, wherein said purge gas is substantially non-absorbent of said projection beam of radiation and comprises two or more components, each component having refractivities at the wavelengths xcex1, xcex2 and xcex3 such that the following equation is substantially fulfilled:                                           α            m1                                (                                          α                m3                            -                              α                m2                                      )                          =                  K          a                                    (        4        )            
wherein xcex1m1 is the refractivity of the purge gas at a wavelength xcex1, xcex1m2 is the refractivity of the purge gas at a wavelength xcex2, xcex1m3 is the refractivity of the purge gas at a wavelength xcex3 and                               K          a                =                              α            a1                                (                                          α                a3                            -                              α                a2                                      )                                              (        5        )            
wherein xcex1a1 is the refractivity of air at a wavelength xcex1, xcex1a2 is the refractivity of air at a wavelength xcex2 and xcex1a3 is the refractivity of air at a wavelength xcex3.
In this aspect, neither the displacement measuring interferometer nor the second harmonic interferometer is required to provide an accurate measurement which takes into account purge gas contamination. Rather, this aspect ensures that the overall interferometric system is adjusted to account for the effect of purge gas contamination. This is achieved by off-setting the errors in the displacement measuring interferometer with those of the second harmonic interferometer. This aspect of the invention provides an alternative manner in which the effects of temperature, pressure and leakage of purge gas may be accurately accounted for in the measurements of the interferometers and allows a simpler mixture of gases, for example only two different gases, to be used.
According to a further aspect of the invention there is provided a lithographic apparatus as specified in the opening paragraph which is characterized by
flushing gas means for supplying purge gas to a space, to displace therefrom ambient air, said space accommodating at least a part of said substrate table and/or at least a part of a table which is a part of said patterning structure, wherein said purge gas is substantially non-absorbent of said projection beam of radiation;
an interferometric displacement measuring means operating at a wavelength xcex1 for measuring the position of said substrate table or the position of said table which is a part of said patterning structure; and
a second harmonic interferometric measuring means operating at wavelengths xcex2 and xcex3 for adjusting the measurements of the said interferometric displacement measuring means (DI) according to the following equation:
L=(DI)xe2x88x92K(SHI)xe2x80x83xe2x80x83(9)
wherein L is the adjusted interferometric displacement measuring means measurement, SHI is the measurement of the second harmonic interferometric measuring means and K is a coefficient, the value of which is optimized such that the effects of variation in pressure, temperature and purge gas composition are partially eliminated from the adjusted measurement L.
In this aspect of the invention, the coefficient K is optimized in order to give the least possible error in the length measurement L. This embodiment is less demanding than the first three embodiments of the invention in terms of the specific combinations of gases which may be used and therefore provides a more cost effective manner in which the errors introduced by the purge gas compositional variation can be reduced.
According to a further aspect of the invention there is provided a device manufacturing method comprising the steps of:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using a radiation system;
using patterning structure to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto a target area of the layer of radiation-sensitive material,
characterized by the steps of
determining the position of a table using interferometric displacement measuring means operating at a wavelength xcex1, said table either being suitable for holding said substrate or forming a part of said patterning structure;
providing purge gas to a space accommodating at least a part of said table to displace therefrom ambient air, wherein said purge gas is substantially non-absorbent of said projection beam of radiation and has a refractive index at a wavelength xcex1 which is substantially the same as that of air when measured at the same wavelength, temperature and pressure.
In a preferred embodiment, the method further comprises the step of adjusting the measurement of said interferometric displacement measuring means to substantially eliminate the effects of variation in pressure and temperature using a second harmonic interferometric measuring means operating at wavelengths xcex2 and xcex3. In this embodiment the purge gas comprises three or more different components, each component having refractivities at the wavelengths xcex2 and xcex3 such that the following equations are substantially fulfilled:                                                         ∑              j                        k                    ⁢                                    F              j                        ⁢                          α              j1                                      =                  α          a1                                    (        1        )                                                                    ∑              j                        k                    ⁢                                    F              j                        ⁡                          (                                                α                  j3                                -                                  α                  j2                                            )                                      =                              α            a3                    -                      α            a2                                              (        2        )            
wherein Fj is the fraction by volume of component j in the purge gas, which purge gas contains a total of k components, xcex1j1 is the refractivity of component j at a wavelength xcex1, xcex1j2 is the refractivity of component j at a wavelength xcex2, xcex1j3 is the refractivity of component j at a wavelength xcex3, xcex1a1 is the refractivity of air at a wavelength xcex1, xcex1a2 is the refractivity of air at a wavelength xcex2 and xcex1a3 is the refractivity of air at a wavelength xcex3, and wherein:                                                         ∑              j                        k                    ⁢                      F            j                          =        1.                            (        3        )            
A further aspect of the invention provides a device manufacturing method comprising the steps of:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using a radiation system;
using patterning structure to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto a target area of the layer of radiation-sensitive material,
characterized by the steps of:
determining the position of a table using interferometric displacement measuring means operating at a wavelength xcex1, said table either being suitable for holding said substrate or forming a part of said patterning structure;
adjusting the measurement of said interferometric displacement measuring means to substantially eliminate the effects of variation in pressure and temperature using a second harmonic interferometric measuring means operating at wavelengths xcex2 and xcex3;
providing purge gas to a space accommodating at least a part of said table to displace therefrom ambient air, wherein said purge gas is substantially non-absorbent of said projection beam of radiation and comprises two or more components, each component having refractivities at the wavelengths xcex1, xcex2 and xcex3 such that the following equation is substantially fulfilled:                                                         ∑              j                        k                    ⁢                      F            j                          =        1.                            (        3        )            
wherein xcex1m1 is the refractivity of the purge gas at a wavelength xcex1, xcex1m2 is the refractivity of the purge gas at a wavelength xcex2, xcex1m3 is the refractivity of the purge gas at a wavelength xcex3 and                               K          a                =                              α            a1                                (                                          α                a3                            -                              α                a2                                      )                                              (        5        )            
wherein xcex1a1 is the refractivity of air at a wavelength xcex1, xcex1a2 is the refractivity of air at a wavelength xcex2 and xcex1a3 is the refractivity of air at a wavelength xcex3.
A further aspect of the invention provides a device manufacturing method comprising the steps of:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using a radiation system;
using patterning structure to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto a target area of the layer of radiation-sensitive material,
characterized by the steps of:
providing purge gas to a space accommodating at least a part of a table to displace therefrom ambient air, said table either being suitable for holding said substrate or forming a part of said patterning structure, wherein said purge gas is substantially non-absorbent of said projection beam of radiation;
determining the position of said table using interferometric displacement measuring means operating at a wavelength xcex1; and
adjusting the measurement of said interferometric displacement measuring means (DI) using a second harmonic interferometric measuring means operating at wavelengths xcex2 and xcex3 according to the following equation:
L=(DI)xe2x88x92K(SHI)xe2x80x83xe2x80x83(9)
wherein L is the adjusted interferometric displacement measuring means measurement, SHI is the measurement of the second harmonic interferometric measuring means and K is a coefficient, the value of which is optimized such that the effects of variation in pressure, temperature and purge gas composition are partially eliminated from the adjusted value L.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
In the present document, the terms illumination radiation and illumination beam are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV, as well as particle beams, such as ion beams or electron beams.